The present invention relates to a process of continuously producing chlorine dioxide. The process comprises reducing chlorate in an acidic reaction medium and, optionally, circulating generator solution between a chlorine dioxide generator and an electrochemical cell. When using an electrochemical cell, the process is performed without crystallization of sulfate and without formation of any solid by-products. The process can also be performed under a wide range of pressures, from vacuum to above atmospheric pressure. In one process alternative, a wide range of reducing agents can be used, including methanol and hydrogen peroxide.
Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes, etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced. Considerable research is also directed to the handling of by-products such as chlorine and mineral acid salts.
There are numerous different processes for chlorine dioxide production. Most processes in commercial use involve reaction of alkali metal (e.g. sodium) chlorate in an acidic medium with a reducing agent. The acidity is generally provided by sulfuric acid. Alkali metal chlorate and sulfuric acid are brought continuously to a reaction vessel to which air and the reducing agent are introduced, usually into the bottom of the vessel. Then chlorine dioxide and air leave from the top of the reaction vessel and a depleted reaction solution is withdrawn for further treatment. It is common to use more than one vessel whereby the depleted reaction solution from the first vessel is brought to a second (and subsequent) vessel together with air and reducing agent for further conversion of the remaining chlorate. The reaction in the reaction vessel(s) is carried out at about atmospheric pressure. Reducing agents used in this type of reaction are sulfur dioxide (the Mathieson process), methanol (the Solvay process) and chloride ions (the R-2 process). The basic chemical reaction involved in the process with chloride ions can be summarized with the formula: EQU ClO.sub.3.sup.- +Cl.sup.- +2 H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O [1]
The other reducing agents are indirect reducing agents, the direct reaction between chlorate ions and methanol or sulfur dioxide being very slow. The direct reducing agent in these cases are chloride ions reacting according to [1]. The chlorine produced is then reacting with methanol to regenerate chloride ions according to the formula: EQU CH.sub.3 OH+3 Cl.sub.2 +H.sub.2 O.fwdarw.6 Cl.sup.- +CO.sub.2 +6 H.sup.+[ 2 ]
or with sulfur dioxide according to the formula: EQU SO.sub.2 +Cl.sub.2 +2 H.sub.2 O.fwdarw.2 HCl+H.sub.2 SO.sub.4[ 3]
As is evident from reaction [1] a large amount of chlorine is produced as a by-product when chloride ions are used as reducing agent. To reduce the amount of chlorine by-product formed in the process, methanol has been used instead of chloride ions as the reducing agent. However, with methanol and also with sulfur dioxide, a certain amount of chlorine is produced since chloride ions are involved in the reduction process. It is also common in these processes to add a small amount of chloride ions, in the form of sodium chloride or hydrochloric acid, to increase the efficiency. Formerly, the chlorine by-product has been utilized in paper mills, but due to increased environmental demands there is a decreasing need for chlorine.
The change over from chloride ions to methanol as the reducing agent also resulted in the disadvantage of formation of by-products other than chlorine in the reaction system. The reaction according to reaction [2] above represents only the theoretical methanol oxidation. However, in practical production, inefficiencies in the methanol oxidation bring about the formation of formaldehyde and formic acid and probably also ethers and esters along with carbon dioxide. It could be expected that reactions can occur in the bleaching train with these by-products thereby resulting in chlorinated organic compounds.
Besides the drawback of formation of chlorine and other by-products, the old R-2, Solvay and Mathieson processes also have the disadvantage of low efficiency and low production rates. The efficiency for a normal Mathieson process calculated as chlorate transformed into chlorine dioxide is typically not more than about 88%.
To increase the efficiency of these processes it has been suggested to run the processes in a single vessel under subatmospheric pressure. Chlorine dioxide is then generated continuously together with the evaporated aqueous reaction medium. The alkali metal sulfate by-product is crystallized. This process is disclosed e.g. in U.S. Pat. No. 4,081,520. This process and similar "single vessel process" ("SVP" process) technologies generally increase the efficiency to acceptable levels while maintaining low levels of chlorine effluent. Patents issued after the above mentioned patent describe different embodiments attempting to optimize the process with as low chlorine production as possible.
Another reducing agent suggested in the prior art for chlorine dioxide production is hydrogen peroxide. U.S. Pat. No. 2,332,181 discloses a batch process for chlorine dioxide production of substantially pure chlorine dioxide with respect to chlorine with hydrogen peroxide as the reducing agent. The process must be run at a low temperature and with low concentrations in the reactor to avoid explosive decomposition. Other patents suggest a combination of hydrogen peroxide and chloride ions as the reducing agent. This combination has the disadvantage of chlorine formation. U.S. Pat. No. 5,091,167 teaches that it is possible to produce chlorine dioxide continuously with high efficiency with hydrogen peroxide as the reducing agent in a chlorine free process with the SVP technology.
However, there is still a need for developing chlorine dioxide processes at atmospheric pressure with good efficiency and production rate but with reduced production of chlorine by-product as well as other by-products. For example, there are a large number of existing plants with atmospheric pressure generators having poor efficiency and capacity limitations. With increasing demand for chlorine dioxide bleaching, improvements of these plants would be of considerable interest. Also, for the installation of new plants, the atmospheric pressure process offers a low investment cost for the chlorine dioxide generator.
Another drawback of known chlorine dioxide processes is the formation of some form of sodium sulfate as a by-product which has to be removed from the reactor, either in the form of a solid saltcake or as waste acid. As mentioned above, most modern processes are operated under subatmospheric pressure, involving precipitation of the sodium sulfate as a saltcake which has to be removed from the reactor by filtering. Today it is hard to find any use for the saltcake and it is normally regarded as an unwanted by-product.
In order to avoid formation of a sulfate by-product, it has been disclosed to provide all acid needed for the chlorine dioxide generation from chloric acid which can be prepared electrochemically from sodium chlorate. Such methods are described in, for example, U.S. Pat. Nos. 4,915,927, 5,084,148 and 5,174,868. However, it has been found difficult to achieve satisfactory current efficiency in production of strong chloric acid which is desirable in order to provide efficient chlorine dioxide generation.
U.S. Pat. No. 4,806,215 discloses a process in which chlorine dioxide is generated from sodium chlorate and hydrochloric acid, in which process the generator liquor is acidified electrochemically and recycled back to the reactor. However, this process necessarily results in co-formation of chlorine which cannot be accepted in modern environmentally friendly bleaching processes.
U.S. Pat. No. 4,129,484 discloses a process of producing chlorine dioxide in which process sulfuric acid and sodium hydrogen sulfate is withdrawn from the reactor and subjected to electrolysis. However, the current efficiency obtained in the electrochemical cell is not satisfactory.
U.S. Pat. Nos. 5,198,080 and 5,122,240 disclose a process of producing chlorine dioxide involving crystallization and withdrawal of solid sodium sesquisulfate and optionally sodium chlorate. The solid salt is dissolved again, electrolytically acidified and recycled to the chlorine dioxide reactor. Since the process involves handling of solid material it is fairly complicated. Further, the sulfate solution obtained by dissolving the solid sesquisulfate is fairly dilute.